Canon EOS 5D MARK IV Instruction Manual page 289

If [z2: Auto Lighting Optimizer] (p.201) is set to any setting other than
[Disable], the image may still look bright even if a decreased flash exposure
compensation for a darker image is set.
If it is difficult to achieve focus with autofocus, the EOS-dedicated, external
Speedlite will automatically emit the AF-assist beam as necessary.
Canon Speedlites Other Than the EX-series
With an EZ/E/EG/ML/TL-series Speedlite set to A-TTL or TTL
autoflash mode, the flash can be fired at full output only.
Set the camera's shooting mode to <a> manual exposure or
<f> aperture-priority AE and adjust the aperture setting before
shooting.
When using a Speedlite that has manual flash mode, shoot in the
manual flash mode.
D Flash Photography
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